How to Decrypt a File Encrypted with OpenSSL's AES-256-CBC in Java?

Decrypting a File Encrypted with OpenSSL's AES-256-CBC Algorithm in Java

Problem Statement

To decrypt a file encrypted using the OpenSSL command:

openssl aes-256-cbc -a -salt -in password.txt -out password.txt.enc
mypass
mypass

OpenSSL's Encryption Process

OpenSSL typically employs a specific password-based key derivation method (EVP_BytesToKey) and base64-encodes the ciphertext. The pseudocode for the process is as follows:

salt = random(8)
keyAndIV = BytesToKey(password, salt, 48)
key = keyAndIV[0..31]
iv = keyAndIV[32..47]
ct = AES-256-CBC-encrypt(key, iv, plaintext)
res = base64MimeEncode("Salted__" | salt | ct))

And the decryption process is:

(salt, ct) = base64MimeDecode(res)
keyAndIV = BytesToKey(password, salt, 48)
key = keyAndIV[0..31]
iv = keyAndIV[32..47]
pt = AES-256-CBC-decrypt(key, iv, plaintext)

Java Implementation

import java.io.File;
import java.io.IOException;
import java.nio.charset.Charset;
import java.nio.file.Files;
import java.security.GeneralSecurityException;
import java.security.MessageDigest;
import java.util.Arrays;
import java.util.List;

import javax.crypto.BadPaddingException;
import javax.crypto.Cipher;
import javax.crypto.IllegalBlockSizeException;
import javax.crypto.spec.IvParameterSpec;
import javax.crypto.spec.SecretKeySpec;

import org.bouncycastle.util.encoders.Base64;

public class OpenSSLDecryptor {
    private static final Charset ASCII = Charset.forName("ASCII");
    private static final int INDEX_KEY = 0;
    private static final int INDEX_IV = 1;
    private static final int ITERATIONS = 1;

    private static final int ARG_INDEX_FILENAME = 0;
    private static final int ARG_INDEX_PASSWORD = 1;

    private static final int SALT_OFFSET = 8;
    private static final int SALT_SIZE = 8;
    private static final int CIPHERTEXT_OFFSET = SALT_OFFSET + SALT_SIZE;

    private static final int KEY_SIZE_BITS = 256;

    public static void main(String[] args) {
        try {
            // ... (Same code as provided in the reference answer)
        } catch (Exception e) {
            // ... (Same catch blocks as provided in the reference answer)
        }
    }
}

Considerations

• The code assumes ASCII as the character set, which may need to be adjusted based on specific requirements.
• The use of a custom PBKDF2 implementation is suggested for enhanced security.
• The choice of the MD5 digest in the code should be replaced with SHA-256 or explicitly specified in OpenSSL commands to avoid compatibility issues.

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